The hydraulic conductivity of geologic materials is an important variable for estimating the rate of transport of contaminants from waste sites. Hydraulic conductivity within earthen material varies with soil water pressure and soil water content.
Prior art instruments for estimating hydraulic conductivity under field conditions fall under two general classes. The first class includes borehole permeameters where water is initially ponded at the bottom of a borehole provided in the earth. Mechanisms are provided for monitoring the liquid level of water within the borehole, and water is added to maintain the liquid in the borehole at a constant level. The rate of water flow required to maintain a constant level of water is utilized to estimate soil hydraulic conductivity at the base of the bore at or near field liquid saturation. This class of instruments also includes modifications whereby the rate of a falling head of liquid within a borehole is monitored, or where an instrument is used at the soil surface and soil infiltration is confined to a ring.
A second class of instruments applies water to the soil surface under negative pressure (i.e., under tension) through a membrane that is permeable to water but not air.
Both classes of instruments operate at or near conditions where the soil is saturated with water and use changes in the volume of water maintained in a reservoir to estimate the soil water fluxes into the soil. Air pressure in the reservoirs is used to regulate the depth of water in a borehole, ring or membrane, and accordingly changes in temperature and atmospheric conditions cause changes that produce errors in the estimated fluxes and pressures. These prior art instruments also require considerable patience and expertise to maintain in the field, and to interpret the resulting data.
Ideal operation of a hydraulic conductivity determining apparatus requires fluxes and pressures to be accurately known. Since soil water pressures and soil water content conditions in the field are often far removed from saturated conditions, an ideal instrument would operate under soil water pressures less then negative 30 kPa and soil water fluxes less then 1 mm/day. The determination of fluxes less than 1 cm/day requires an ideal instrument to operate unattended for several days in the field to reach a steady flux and soil water pressure.
It would be desirable to overcome these and other drawbacks associated with the prior art in the development of an apparatus and improved methods for determining hydraulic conductivity in earthen material.